1. Field
The present invention relates to an apparatus and method for characterizing glass sheets, and in particular to an apparatus adapted to measure one or more selected attributes of a glass sheet while the glass sheet is in motion.
2. Technical Background
The invention relates to controlling motion and attitude of a glass sheet being conveyed along a predetermined path to enable high resolution online measurements, such as topography inspection (e.g. nanotopography, or topography on a nanometer scale). Accurate online measurements of thin glass sheets, and in particular the measurement of nanometer-scale features, is highly dependent upon consistently presenting the glass sheets in a predetermined plane at a predetermined orientation and eliminating the vast majority of vibrations and oscillations of the glass. Challenges such as high accuracy measurement of thickness, waviness, and cord are highly dependent on presenting the glass surface to measurement gauges with reproducible, high tolerance material handling.
Online process control and quality measurements, such as online cord and streak inspection, suffer from a lack of repeatability when performed while simply gripping the glass in a non-quality area. A large part of the material presentation challenge is keeping the sheet of glass moving down a production line while being suspended from a carrier on an overhead conveyor. This constraint often forces measurements to be performed either in a separate inspection portion of the process line, completely offline, or the measurement technology compatible with coarse traditional online material handling is limited in its performance (e.g. resolution).
High resolution metrology is performed online in other industries, such as silicon wafers or paper and plastic webs, but in these cases, the product is in direct contact with a supporting plate, as with wafers, or with rollers, as with most webs. The size and contact prohibition on glass sheets suitable for display applications presents a difficult challenge for handling.